1. Field of the Invention
This invention relates to a digital signal processing apparatus for muting a digital signal digitized with at least one bit, and a digital signal recording apparatus for recording the digital signal or the muted signal on a recording medium.
2. Description of the Related Art
As a method for digitizing audio signals, a method known as a sigma-delta (xcexa3xcex94) method has become known (see for example Y. Yamazaki, xe2x80x9cAD/DA Converter and Digital Filterxe2x80x9d, Journal of Japan Society of Acoustics, Vol.46, No.3 (1990), pp. 251-257).
The 1-bit digital data, obtained by this xcexa3xcex94 modulation, is represented by a sampling frequency significantly higher (such as 44.1 kHz times 64) and a data word length significantly shorter (such as 1 bit) than those of the data format hitherto used for conventional digital audio (such as the sampling frequency of 64 kHz and the data word length of 16 bits) and is characterized by a broad transmission frequency range. By this xcexa3xcex94 modulation, a high dynamic range can be secured in an audio range which is an extremely low frequency range as compared to the 64-tuple oversampling frequency. This feature can be exploited for recording or data transmission with high sound quality.
The xcexa3xcex94 modulation circuit itself is not a new technique and is frequently used in an A/D converter since the circuit structure can be easily designed as an IC and lends itself easily to high-precision A/D conversion.
In case of 1-bit xcexa3xcex94 modulation, an audio signal is represented by a bi-level pulse density modulation of xe2x80x9c+1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d. At this time, a xcexa3xcex94 signal Xn at a point equivalent to a neutral point voltage (that is, 0 V) as a reference for an audio signal is represented by the equation (1):                                           ∑                          n              =              0                        m                    ⁢                      xe2x80x83                    ⁢          Xn                =                                                            ∑                                  n                  =                  0                                                  m                  =                                      a                    +                    b                                                              ⁢                              xe2x80x83                            ⁢                                                (                                      +                    1                                    )                                xc3x97                a                                      +                                          (                                  -                  1                                )                            xc3x97              b                                =                                    0              →              a                        =            b                                              (        1        )            
As will be clear from the equation (1), a and b are selected so that, if the neutral point potential is to be represented, the pulse density for xe2x80x9c+1xe2x80x9d and that for xe2x80x9cxe2x88x921xe2x80x9d will be equal to each other.
If the 1-bit xcexa3xcex94 modulated signal is actually recorded/reproduced or transmitted as a digital signal, xe2x80x9c+1xe2x80x9d is converted into xe2x80x9c1xe2x80x9d, while xe2x80x9cxe2x88x921xe2x80x9d is converted into xe2x80x9c0xe2x80x9d. If an unusual event occurs in the course of recording and reproduction or if the transmission path is ruptured such that the signal is lost, the signal is fixed at xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. Since a series of contiguous xe2x80x9c1xe2x80x9d s or xe2x80x9c0xe2x80x9ds in xcexa3xcex94 modulation is equivalent to a positive maximum value or to a negative maximum value, respectively, there is a risk that, at a time moment of occurrence of an unusual event in the course of recording/reproduction or rupture of the transmission path, the noise of the maximum output is generated to destruct an amplifier for monitoring or a speaker.
The digital audio recorder for recording the xcexa3xcex94 modulated digital audio signal and the peripheral equipment are of a higher dynamic range and of a broader transmission frequency range than before in order to realize high flexibility for coping with a variety of digital audio media which will be developed in the future.
Meanwhile, for realization of a digital audio recorder for recording xcexa3xcex94 modulated digital audio signals and the peripheral equipment, the function of noise prevention and the muting function for protecting downstream side equipment in case of an occurrence of signal interruption as a result of a malfunction of the equipment, damage to the recording media or rupture of the transmission path, are indispensable. However, there has hitherto lacked a method for muting in the digital signal processing system handling xcexa3xcex94 modulation.
It is therefore an object of the present invention to provide a digital signal processing apparatus capable of muting at least a one-bit digital signal and a digital signal recording apparatus capable of recording at least the one-bit digital signal or muted signal.
In one aspect, the present invention provides a digital signal processing apparatus for muting a digital signal obtained on digitizing in at least one bit. The apparatus includes muting pattern signal generating means for generating a pre-set muting pattern, switching means for selectively switching between the digital signal digitized with at least one bit and the muting pattern signal generated by the muting pattern signal generating means, and an finite response filter means for converting an output of the switching means into an analog signal. The spectrum characteristics of the pre-set muting pattern signal are selected to coincide with the inhibiting frequencies of the finite response filter means.
In another aspect, the present invention provides a digital signal recording apparatus for muting a digital signal obtained on digitizing in at least one bit in a recording medium. The apparatus includes muting pattern signal generating means for generating a pre-set muting pattern, switching means for selectively switching between the digital signal digitized with at least one bit and the muting pattern signal generated by the muting pattern signal generating means, finite response filter means for converting an output of the switching means into an analog signal, and recording means for recording an output signal of the switching means on a recording medium. The spectrum characteristics of the pre-set muting pattern signal are selected to coincide with the inhibiting frequencies of the finite response filter means.
In yet another aspect, the present invention provides a digital signal muting method for muting a digital signal digitized with at least one bit, based on a muting command, at the time of transmission of said digital signal. The method includes the steps of selecting a muting signal generated by the muting command so that the spectrum distribution of the muting signal will coincide with the inhibiting frequencies of an finite response filter configured for converting the digital signal into an analog signal.
With the digital signal processing apparatus according to the present invention, the switcher switches between the muting pattern signal generated by the muting pattern generating means and at least one-bit digital signal, and the output selected by this switching is converted by an FIR filter into an analog signal. Since the muting pattern signal has plural inhibiting frequencies corresponding to the inhibiting frequencies of the FIR filter so that the analog signal will be at a neutral point potential after analog modulation, the 1-bit digital signal can be muted when the muting pattern signal is selected by the switcher.
With the digital signal recording apparatus according to the present invention, the switcher switches between the muting pattern signal generated by the muting pattern generating means and at least one-bit digital signal, and the output selected by this switching is recorded on the recording means, at the same time as it is converted by an FIR filter into an analog signal. Since the muting pattern signal has plural inhibiting frequencies corresponding to the inhibiting frequencies of the FIR filter so that the analog signal will be at a neutral point potential after analog modulation, the muted 1-bit digital signal can be recorded when the muting pattern signal is selected by the switcher.